In recent years, electronic devices are rapidly becoming more portable and cordless. For use as a driving power source for such devices, there is an increasing demand for small-size and light-weight secondary batteries with high energy density. Moreover, characteristics such as high output characteristics, durability over a long period of time, and safety are required not only for small-size secondary batteries for consumer use, but also for large-size secondary batteries for use in power storage apparatus and electric vehicles. Among secondary batteries, non-aqueous electrolyte secondary batteries with high voltage and high energy density are being developed actively.
As for lithium ion secondary batteries representing non-aqueous electrolyte secondary batteries, the positive electrode active material is required to have a high capacity density and good reversibility in a high voltage range. For this reason, lithium-containing transition metal composite oxides such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and lithium nickel manganese cobalt oxide (LiNi1/3Mn1/3Co1/3O2) have been used as the positive electrode active material.
When non-aqueous electrolyte secondary batteries are stored at high temperatures, problematically, gas generation occurs at the surface of the positive electrode active material, causing the battery to swell. In particular, in an attempt to meet the demand for higher energy densities in recent years, there is a case where the density of active material is increased so that the ratio of the space occupied by the active material within the battery can be increased. In this case, the space allowing the generated gas to enter is reduced, and the swelling of the battery becomes more noticeable. Conventionally, such gas generation has been considered as being by-produced in the production process of a positive electrode active material or positive electrode, and as being attributed to the presence of lithium carbonate and the like in the vicinity of the positive electrode active material, and various attempts have been made to reduce them.
For example, Patent Literature 1 proposes that the amount of lithium carbonate present on the surface of a lithium nickel composite oxide be set to 0.20 mass % or less, relative to the lithium nickel composite oxide, and a porous layer containing an inorganic oxide and lithium carbonate be provided on the surface of the positive electrode.
Patent Literature 2 proposes that a phosphorus compound be contained in the positive electrode active material near its surface by treating the positive electrode active material with phosphorous acid or a phosphoric acid compound. Patent Literature 2 also proposes that the concentration of carbonate and hydrogencarbonate in the positive electrode active material be set to 0.30 wt % or less.
As a proposal for improving the safety, for example, Patent Literature 3 suggests that lithium carbonate be present concentratedly at the surface of active material. The purpose of employing this method in Patent Literature 3 is to cause gas generation effectively during overcharge so that the safety valve can operate more reliably. In Patent Literature 3, the content of lithium carbonate in the active material layer is set to 0.3 wt % or more in order to maximize the effect of adding lithium carbonate.